While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-UV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F, laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004).
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for lithography using KrF excimer laser, photoacid generators capable of generating alkane- or arene-sulfonic acid are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography results in an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution or a low sensitivity. Thus these photoacid generators are not suited for the fabrication of microelectronic devices.
For the above reason, photoacid generators capable of generating perfluoroalkanesulfonic acid having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkanesulfonic acid have already been developed for use in the KrF resist compositions. For instance, JP-A 2000-122296 and U.S. Pat. No. 6,048,672 (or JP-A 11-282168) describe photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. JP-A 2002-214774, US Patent Application Publication 2003-0113659 A1 (JP-A 2003-140332), and US Patent Application Publication 2002-0197558 A1 describe novel photoacid generators capable of generating perfluoroalkyl ether sulfonic acids.
On the other hand, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their stability (or non-degradability) due to C—F bonds, and biological concentration and accumulation due to hydrophobic and lipophilic natures. The US EPA adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further listing 75 chemical substances although their use in the photoresist field is excluded. It has already been proposed to apply the Rule to perfluoroalkanesulfonic acids and derivatives thereof, summing to 183 chemical substances.
Facing the PFOS-related problems, manufacturers made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution. For instance, JP-A 2004-531749 describes the development of α,α-difluoroalkanesulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon irradiation, specifically di(4-tert-butylphenyl)-Iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate. JP-A 2004-002252 describes the development of α,α,β,β-tetrafluoro-alkanesulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2002-214774 discloses such photoacid generators having difluorosulfoacetic acid alkyl esters (e.g., 1-(alkoxycarbonyl)-1,1-difluoro-methanesulfonate) and difluorosulfoacetic acid amides (e.g., 1-carbamoyl-1,1-difluoromethanesulfonate) although their synthesis method is lacking. Furthermore, JP-A 2005-266766 discloses a photosensitive composition comprising a compound capable of generating a partially fluorinated alkane sulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
The substances disclosed in these patent documents have a reduced degree of fluorine substitution, but suffer from several problems. They are less degradable because they are based on substantially undegradable hydrocarbon skeletons and they do not possess readily degradable substituent groups such as ester groups. A certain limit is imposed on the molecular design for changing the size of alkanesulfonic acid. The starting materials containing fluorine are expensive.
Aside from the degradation and acid strength of photoacid generators, there are many problems to be solved. For instance, as the pattern layout becomes finer, the fluctuation of pattern line width, known as “line width roughness” (LWR), becomes significant. In the processing of gate electrode zones in the LSI circuit manufacturing process, for example, poor LWR can give rise to such problems as current leakage, resulting in a transistor with degraded electrical properties. It is believed that the LWR is affected by various factors. The main factor is the poor affinity of a base resin to a developer, that is, low solubility of a base resin in a developer. Since carboxylic acid protective groups commonly used in the art are bulky tertiary alkyl groups and thus highly hydrophobic, most of them are less soluble. Where a high resolution is required as in the formation of microscopic channels, a noticeable LWR can lead to an uneven size. One of known approaches for reducing LWR is by increasing the amount of photoacid generator added, as described in Journal of Photopolymer Science and Technology, Vol. 19, No. 3, 327-334 (2006). This approach, however, exerts a less than satisfactory effect, sometimes at the substantial sacrifice of exposure dose dependency, mask fidelity and/or pattern rectangularity. Aside from merely increasing the amount, uniform distribution of the photoacid generator is also important for improving LWR.
Furthermore, as the circuit line width is reduced, the detrimental influence of acid diffusion on contrast becomes more serious for resist materials. This is because the pattern size is approaching the diffusion length of acid. The acid diffusion may also lead to a lowering of mask fidelity and a degradation of pattern rectangularity. Therefore, in order to take full advantage of the reduced wavelength of a light source and the increased NA, it is necessary to increase the dissolution contrast over prior art materials or to restrain the acid diffusion.
JP-A 2007-145797 discloses C1-C20 alkanecarbonyloxy or arenecarbonyloxy-1,1,3,3,3-pentafluoropropanesulfonates, typically triphenylsulfonium 2-(adamantane-1-carbonyloxy)-1,1,3,3,3-pentafluoropropanesulfonate. They are still insufficient in diffusion control and LWR reduction.
JP-A 2007-161707 and JP-A 2008-069146 disclose photoacid generators capable of generating partially fluorinated alkanesulfonic acids having a polycyclic hydrocarbon group. These compositions still fail to exert satisfactory resist performance. Since these photoacid generators are unstable esters of difluoroacetic acid, shelf stability is a matter of concern about the resist compositions comprising the same.
In the actual device fabrication process, the exposure dose can vary in a certain range. The resist is required to have an exposure latitude sufficient to maintain a pattern profile substantially the same even when a certain variation of exposure dose occurs. Under the current demand for a further miniaturization of the pattern rule, the resist is required to perform well with respect to sensitivity, substrate adhesion, and etch resistance, and additionally, to achieve an LWR improvement and exposure latitude without the concomitant degradation of resolution.
With respect to the immersion lithography, there remain some problems. Minute water droplets are left on the resist and wafer after the immersion exposure, which can often cause damages and defects to the resist pattern profile. The resist pattern after development can collapse or deform into a T-top profile. In the immersion lithography, there exists a need for a patterning process which can form a satisfactory resist pattern after development.